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“As you would expect, we have tight quality controls in place, however variances will always occur in a small number of coins, particularly in the striking process, due to the high volumes and speed of production.”

Counterfeiting of the old £1 has led to about one in 40 being fake.

Defective new coins are now being sold on eBay by people hoping to cash in on the rare occurrence.

Philip Munsell, director of Coin News magazine, told the BBC: “The thing about the Royal Mint is that they are very, very good with their quality control, so therefore if you have something that has passed through without being quality checked then it is likely to be collectable, it is likely to therefore be worth more than one pound.”